It is proposed to develop a model system for studying the mutagenesis of alkylating carcinogens in asynchronous and synchronous mammalian cells in vitro. Using asynchronous cells of different species and tissue origin, we will measure the production of mutations by ethylnitrosourea (ENU) and related environmental carcinogens. We will determine the effects of cellular repair processes on the induction of mutations by measuring the loss of carcinogen-produced damage and relating it to mutagenesis. In addition, we will measure the induced frequency of certain specific mutations during the cell cycle to test if specific mutagenic changes are associated with specific cell-cycle times. ENU is used because of its advantageous chemical characteristics, and because of its special carcinogenic properties. Exposing homogeneous populations of selectively synchronized mammalian cells to this agent offers a novel way to test for alkylation-induced mutations in mammalian cells. These experiments may give us a practical understanding of chemical mutagenesis from alkylating agents which will be useful for estimating total risks, as well as risks relative to other environmental carcinogens to the human population.